Since 2008, the U.S. Embassy in Beijing has issued hourly pollution readings of small airborne particles known as PM 2.5 on the Twitter feed @beijingair, a service has since spread to U.S. consulates in Guangzhou and Shanghai. Now, after years of private complaints, China has publicly demanded the U.S. stop the practice. Vice Minister of Environmental Protection Wu Xiaoqing told a press conference Tuesday that foreign embassies posting information about Chinese air quality was a violation of Chinese law and the Vienna Convention on Diplomatic Relations. “Diplomats are obligated to respect and abide by the laws and regulations in the receiving states. In addition, they cannot interfere with the domestic issues of receiving states,” Wu said. “We wish those embassies and consulates will respect China’s laws and stop publishing air quality data which is not representative.”

Despite the Chinese complaints, the U.S. does not plan to stop reporting its air-quality data, Mark C. Toner, a deputy State Department spokesperson, said at a briefing Tuesday. “We provide the American community, both our Embassy and consulate personnel, as well as the American community writ large, information it can use to make better daily decisions regarding the safety of outdoor activities,” he said. Toner described the readings as “information that Americans get in U.S. cities every day.”

China first complained privately about the U.S. embassy’s air-quality monitoring in 2009, telling U.S. diplomats in a sometimes heated meeting that it might cause confusion among the Chinese public and lead to “social consequences,” according to a cable released by Wikileaks. At that time the U.S. said that such reporting was necessary because China had yet to begin releasing information about PM 2.5 levels to the public or posting any real-time data about air quality. The U.S. said the Embassy began monitoring pollution to help its staff and their families plan activities. Under a regulation known as “no double standard,” the U.S. decided that it had to make the information public. The double standard rule was put in place after the 1988 Pan Am Flight 103 bombing over Lockerbie, Scotland, when the U.S. State Department had information that a flight from Germany to the U.S. might be attacked — information that was shared within some U.S. embassies but not revealed to the public.

Chinese officials may understandably resent the implied comparison of Beijing’s air quality to a terrorist attack, but the danger is quite real. Last winter, the state-run China Daily newspaper reported that the lung cancer rate in the Chinese capital had climbed by 60% over the past decade even as levels of smoking had fallen, leaving air pollution as a primary culprit. Awareness of the risks has climbed dramatically in recent months. On Beijing’s streets this spring, there has been an explosion of cyclists and pedestrians wearing high-quality air masks. And local environmentalists have begun doing air-pollution monitoring of their own.

The U.S. air-quality readings have played a role in raising local interest in pollution issues. Twitter has been blocked in China since 2009, but several applications for iPhones and Android mobile devices now allow users to easily post the @beijingair data on Sina Weibo, the popular Chinese Twitter-like service. The U.S. has also begun publishing air-quality data on the websites of the U.S Embassy in Beijing and consulates in Shanghai and Guangzhou, which aren’t blocked in China. Last year, several prominent Weibo users began questioning the differences between the U.S. data and Beijing’s own daily reports on levels of larger PM 10 particles, sulfur dioxide and nitrogen dioxide.

That helped push Beijing’s Environmental Protection Bureau to announce early this year that it would begin reporting hourly results for PM 2.5 and ozone levels. The city’s plan is to set up a PM 2.5 monitoring network by the end of the year, and so far it is just publishing data from a single measuring station. The information is given as raw quantitative numbers, instead of the kind of qualtitative descriptions that appear on the U.S. Embassy’s Twitter feed and website, which tell residents whether they need to worry about the latest reading.

So, for now, Beijing residents are likely to continue to rely on the U.S. reports, which describe conditions ranging from good to hazardous. At one particularly polluted point in 2010, the reading was described as “crazy bad,” a term that has now been replaced with the more sober “beyond index.” While the U.S. data is only from a single monitor atop the U.S. Embassy, which both the U.S. and China say is insufficient for a complete picture of the city’s air, it does give a snapshot of pollution at the moment that’s useful for anyone planning to go outside.